The present invention relates to a method and apparatus for separating air to produce an oxygen product. More particularly, the present invention relates to such a method and apparatus in which air is separated in double column arrangement having higher and lower pressure columns. Even more particularly, the present invention relates to such a method and apparatus in which the lower pressure column is reboiled with compressed nitrogen vapor from the higher pressure column and the vapor rate is increased at an intermediate location thereof by generation of vaporized liquid.
Air is commonly separated in a double column arrangement having higher and lower pressure columns. Prior to separation, air is filtered and compressed. After removing the heat of compression, the air is purified by removing impurities such as carbon dioxide, moisture and heavy hydrocarbons. The resultant compressed and purified air stream is then cooled in a main heat exchanger to a temperature suitable for its rectification and introduced into double column arrangement. Liquid oxygen is produced as a column bottoms of the lower pressure column. An oxygen product is extracted as a liquid stream that may be pumped to pressurize the liquid. The liquid is then vaporized in the main heat exchanger against cooling the incoming air.
In order to reboil the lower pressure column, a condenser reboiler can be provided to condense incoming air against boiling the liquid oxygen. The air may be partially or fully condensed and is introduced into the higher pressure column. Examples of this can be found in U.S. Pat. No. 5,626,036 and WO 885893. In both of these patents the air is partially condensed to reboil the lower pressure column. Such partial condensation is advantageous in that the majority of the air may be compressed in the main compressor to a pressure below 4 bar absolute. This minimum compression will produce a minimum amount of boiling in the lower pressure column so that a liquid product may be withdrawn. Additionally, in both of these patents, an increase in the vapor rate is effected at an intermediate location of the lower pressure column by means of an intermediate reboiler in which nitrogen vapor constitutes the coolant. The condensate of such intermediate reboiler is returned to both the higher and lower pressure columns as reflux.
As will be discussed, the present invention produces greater efficiency than such prior art patents.